Bump pytest req to <5 since we got it working for 4.x

setup.py

@@ -24,11 +24,7 @@
         # TODO: do we need to name the LHS 'pytest_relaxed' too? meh
         "pytest11": ["relaxed = pytest_relaxed.plugin"]
     },
-    install_requires=[
-        "pytest>=3,<4",
-        "six>=1,<2",
-        "decorator>=4,<5",
-    ],
+    install_requires=["pytest>=3,<5", "six>=1,<2", "decorator>=4,<5"],
     classifiers=[
         "Development Status :: 5 - Production/Stable",
         "Framework :: Pytest",